1. Field of the Invention
This invention relates to the protection of telephones from faults causing excessive current and power dissipation.
2. Description of Related Art
When the handset of a telephone is lifted, a line switch inside the telephone closes and allows current from the exchange to flow through the telephone. Modern telephones, especially hands-free telephones, often use a semiconductor (such as a bipolar or field effect transistor) to provide the switching of the current. With normal exchange battery voltages and feed impedances, the semiconductor switch remains saturated when closed and very little power is dissipated in the switch. During abnormal conditions such as an exchange ring trip failure or earth fault, an excessive level of current may flow through the telephone and damage the components of the telephone. If a semiconductor switch is used, the overload can be detected and the semiconductor switch can be used to limit the current to a predetermined level.
A problem with this arrangement is that the semiconductor line switch will come out of saturation and a significant voltage drop appears across the semiconductor line switch. Although the current has been limited to a safe level, the power dissipated by the line switch is the product of this current and the voltage drop across the line switch. The power level can easily exceed the rating of the semiconductor line switch and cause a failure of the semiconductor line switch.